Support @Convert specification on id attributes

We want to map an id attribute to an immutable type.

```java
class FooId implements Serializable {
private final int value;

private FooId(int value) {
this.value = value;
}

public static FooId of(int value) {
return new FooId(value);
}

public int value() {
return value;
}

@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
FooId fooId = (FooId) o;
return value == fooId.value;
}

@Override
public int hashCode() {
return Objects.hash(value);
}
}

class Converter implements AttributeConverter {

@Override
public Integer convertToDatabaseColumn(FooId attribute) {
return Optional.ofNullable(attribute).map(FooId::value).orElse(null);
}

@Override
public FooId convertToEntityAttribute(Integer dbData) {
return Optional.ofNullable(dbData).map(FooId::of).orElse(null);
}
}

@Entity
class Foo {
@Id
@Convert(converter = Converter.class)
@Column(definition = "integer")
private FooId id;
}
```

Hibernate 5 fails to convert the id to the immutable type.

Reading the JPA 2.1 spec, we found this:

>The Convert annotation should not be used to specify conversion of the following: Id attributes
(including the attributes of embedded ids and derived identities), version attributes, relationship
attributes, and attributes explicitly annotated (or designated via XML) as Enumerated or Temporal. Applications that specify such conversions will not be portable

Would it be possible to reconsider this to make `convert on id attributes` a use case part of the next version of JPA?

Start by reviewing the issue's immutable FooId and AttributeConverter example alongside the quoted JPA 2.1 restriction on Id attributes. Determine whether Jakarta Persistence should support @Convert on id attributes and what specification change would be required; completion would require a decided, actionable scope rather than only reconsideration.

Written by the indexing model from the issue text.